The present invention relates to a method of manufacturing a multi-layered wiring board comprising plurality of inner circuit plates with an adhesive insulating layer interposed therebetween, and more particularly to a method of manufacturing a multi-layered wiring board, which is effective in preventing a defective adhesion between a wiring layer consisting of a copper foil, which is formed on the inner circuit plate, and the adhesive insulating layer, and in preventing treatment agents to be used in the manufacturing process from remaining at an interface between the wiring layer and the adhesive insulating layer. 2. Description of the Related Art
The conventional multi-layered wiring board has been manufactured by a series of steps as shown in FIGS. 1 to 7. Namely, as shown in FIG. 1, a plurality of inner circuit plate "a" having a wiring layer consisting of a copper foil are integrally laminate together with an outer copper foil "al" with an adhesive insulating layer (prepreg) "b" being interposed therebetween. Then, the resultant composite is drilled to form a through hole "c" as shown in FIG. 2. In order to cover the inner wall surface of the through hole "c" with a copper foil, a plating treatment is performed to form a plated layer "d" on the inner surface of the through hole "c" as well as on the surface of the laminated composite as shown in FIG. 3.
Then, a pattern of a photoresist layer "e" is formed over the surface of the plated layer "d" except a surrounding portion of the through hole "c" and a wiring layer-forming portion as shown in FIG. 4. On this exposed portion which is not covered by the photoresist layer "e" is deposited a copper solder plating layer "f" as shown in FIG. 5. After removing the photoresist layer "e", the plated layer "d" is etched off using the copper solder plating layer "f" as a mask as shown in FIG. 6. Finally, a solder-resist layer "g" is formed on a predetermined surface of the laminated composite as shown in FIG. 7, thereby obtaining a multi-layered wiring board "h".
Since the wiring layer formed On the inner circuit plate "a" is constituted by a copper foil of smooth surface, the bonding strength between the inner circuit plate "a" and the adhesive insulating layer (prepreg) "b" is insufficient when a plurality of inner circuit plate "a" are laminated one upon another by interposing the adhesive insulating layer (prepreg) "b" therebetween as mentioned above, so that the peeling between the wiring layer and the adhesive insulating layer "b" is caused to occur increasingly with time.
In order to avoid this problem, there is proposed to make ragged the surface of the wiring layer "i" consisting of a copper foil by oxidizing the surface thereof by using an aqueous solution of alkaline sodium cholite containing for example 15-25 g/l of sodium hydroxide (NaOH), thereby forming a needle crystalline oxide film "j" consisting of CuO and Cu.sub.2 O.
Through this treatment, the bonding strength between the wiring layer "i" and the adhesive insulating layer "b" can be improved as a whole. However, since the oxide film "j", which is resistive to an alkaline solution, is relatively easily dissolved into an acidic solution, when the exposed laminated surface "k" on the inner wall of the through hole "c" is exposed, as shown in FIG. 9A, to an acidic treatment agent in the manufacturing process of the multi-layered wiring board (for example, when the through hole "c" is treated with an acidic aqueous solution of palladium/tin containing hydrochloric acid in the plating step for making the through hole "c" catalytically sensitivity to an electrolytic copper plating) as shown in FIG. 9A, the oxide film "j", which is contacted with the acidic solution, is dissolved by the acidic solution, so that a portion of copper of the wiring layer "i" is exposed as shown in FIG. 9b. As a result, a phenomenon of so-called "haloing", which is a pink ring "r" in shape, is caused to occur around the through hole "c" as shown in FIG. 10.
When this phenomenon is caused to occur, a space "s" is formed as a result at an interface between the wiring layer "i" and the adhesive insulating layer "b" on the inner wall of the through hole "c", thus inviting the deterioration of the bonding strength of this interface portion, and raising a problem of peeling with time. Further, the treatment agent may be easily intruded into this space "s", and tends to be kept therein, thus markedly degrading the reliability of the product as a multi-layered wiring board.
It has been found out by the present inventors that the reason of why the oxide film "j" formed in accordance with the conventional method does not show a sufficient acid resistance is attributed to the presence of a lot of CuO, which is easily dissolved by an acid, and that if this CuO is diminished, while increasing the ratio of Cu.sub.2 O which is relatively resistive to acid, the acid resistance of the oxide film "j" can be highly improved. This invention has been accomplished on the basis of above findings.
Meanwhile, the oxide film as obtained according to the conventional method indicates the ratio of Cu.sub.2 O to CuO (Cu.sub.2 O/CuO.times.100) as being in the range of about 30-40%, and the breadth of the haloing to be formed in the conventional method (the width of the ring, as indicated by a distance "L" in FIG. 10 as measured from the brim of the through hole 0.35 mm in diameter) as being in the range of 130-150 .mu.m or more, in some case several hundreds micrometers.